Preheat oven to 350F degrees. Peel and boil yams until mashable. Add butter, cinnamon, ginger, salt, nutmeg and sugar to hot yams. Beat until light and smooth. Beat egg yolks until light; add to mixture. Stir in orange juice, rind and milk. Beat egg whites until stiff, and fold in. Pour mixture into unbaked pie shell. Bake 35 minutes or until pie puffs up and is firm in the middle. Cool on rack. Add whipped cream.